Nitride semiconductor device

ABSTRACT

A nitride semiconductor device includes a semiconductor multilayer formed on a substrate, a first ohmic electrode and a Schottky electrode spaced apart from each other on the semiconductor multilayer; and a passivation film covering a top of the semiconductor multilayer. The semiconductor multilayer  102  includes a first nitride semiconductor layer, a second nitride semiconductor layer, and a p-type third nitride semiconductor layer  124  sequentially formed on the substrate. The third nitride semiconductor layer contains p-type impurities, and is selectively formed between the first ohmic electrode and the Schottky electrode in contact with the Schottky electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of PCT International ApplicationPCT/JP2010/002981 filed on Apr. 26, 2010, which claims priority toJapanese Patent Application No. 2009-203978 filed on Sep. 3, 2009. Thedisclosures of these applications including the specifications, thedrawings, and the claims are hereby incorporated by reference in theirentirety.

BACKGROUND

The present disclosure relates to nitride semiconductor devices, andmore particularly to nitride semiconductor devices which can be used aspower devices in power supply circuits etc.

Nitride semiconductors represented by gallium nitride (GaN) are wide gapsemiconductors. For example, GaN has a bandgap of 3.4 eV at roomtemperature. Aluminum nitride (A1N) has a bandgap of 6.2 eV at roomtemperature. Also, nitride semiconductors have high breakdown fieldstrength and high saturated drift velocity of electrons, as compared tocompound semiconductors such as gallium arsenide (GaAs) etc., or silicon(Si) semiconductors, etc. In a heterostructure (AlGaN/GaNheterostructure) of aluminum gallium nitride (AlGaN) and GaN, chargesare generated at a heterointerface due to spontaneous polarization andpiezoelectric polarization. A sheet carrier concentration of 1×10¹³ cm⁻²or more is obtained even when AlGaN and GaN are undoped. A diode and ahetero junction field effect transistor (HFET) having high currentdensity can be provided by utilizing two-dimensional electron gas (2DEG)generated at the heterointerface. Therefore, power devices etc. made ofnitride semiconductor which is advantageous in increasing output powerand a breakdown voltage are now being actively researched and developed.

A Schottky diode is one type of diode used as a power device. A Schottkydiode having an AlGaN/GaN heterostructure operates with a large currentand low resistance, since two-dimensional electron gas generated at theinterface between an undoped AlGaN layer and an undoped GaN layer isused as a channel. In general, a Schottky diode has excellent switchingcharacteristics and a low forward threshold voltage as advantages. Adisadvantage is, however, that a reverse leakage current is large.

In order to reduce a reverse leakage current of a Schottky diode, amethod, in which two types of metal are used for an anode electrode, andthe metal with a higher Schottky barrier is formed to cover the metalwith a lower Schottky barrier, is suggested. (See, for example, JapanesePatent Publication No. 2005-317843). When a forward-direction voltage isapplied to the diode, a current flows to the metal with the lowerSchottky barrier, and a low threshold voltage can be thus maintained. Onthe other hand, when a reverse-direction voltage is applied to turn thediode off, the metal with the higher Schottky barrier reduces thereverse leakage current.

A passivation film is usually formed on a surface of a device as asurface protection film. The formation of the passivation film providesthe advantages of reducing formation of a surface state and reducing thephenomenon called “current collapse” of reduction in a forward current.Since it has also the function of protecting the device from impurities,the passivation film needs to be formed in view of improving reliabilityof the device.

SUMMARY

However, the present inventors found the problem that reverse leakagecurrents of a diode largely increase if a passivation film is formed.This may be because a leakage path is formed at the interface betweenthe passivation film and an undoped AlGaN layer. When the leakage pathis formed at the interface between the passivation film and the undopedAlGaN layer, reverse leakage currents cannot be reduced, even if thestructure of an anode electrode is innovated. Not only a diode, but alsoa hetero junction field effect transistor (HFET) etc. have a similarproblem.

Based on the findings of the present inventors, it is an objective ofthe present disclosure to provide a nitride semiconductor device withreduced interface leakage currents even when a passivation film isformed.

In order to achieve the objective, the present disclosure provides anitride semiconductor device including a p-type nitride semiconductorlayer formed between a Schottky electrode and an ohmic electrode incontact with the Schottky electrode.

Specifically, an example nitride semiconductor device includes asemiconductor multilayer including at least one first nitridesemiconductor layer formed on a substrate, at least one second nitridesemiconductor layer formed on the first nitride semiconductor layer, andhaving a wider bandgap than the first nitride semiconductor layer, and ap-type third nitride semiconductor layer selectively formed on thesecond nitride semiconductor layer; a first ohmic electrode and aSchottky electrode spaced apart from each other on the semiconductormultilayer; and a passivation film covering a top of the semiconductormultilayer. The third nitride semiconductor layer is formed between thefirst ohmic electrode and the Schottky electrode in contact with thesecond nitride semiconductor layer and the Schottky electrode.

In the example nitride semiconductor device, the third nitridesemiconductor layer is formed between the first ohmic electrode and theSchottky electrode in contact with the second nitride semiconductorlayer and the Schottky electrode. Thus, the leakage path appearing atthe interface between the semiconductor multilayer and the passivationfilm is blocked by the third nitride semiconductor layer. In addition,Schottky leakage can be reduced, since a depletion layer expands fromthe third nitride semiconductor layer. Thus, even when the passivationfilm is formed, leakage currents can be largely reduced as compared to aconventional nitride semiconductor device. On the other hand, when aforward bias voltage is applied, the size of the depletion layerdecreases and has little influence on forward characteristics.

In the example nitride semiconductor device, the Schottky electrode maybe in contact with at least part of an upper surface of the thirdnitride semiconductor layer. This structure facilitates alignment whenforming the Schottky electrode.

In the example nitride semiconductor device, the second nitridesemiconductor layer may include a raised portion with a greaterthickness than the other portions of the second nitride semiconductorlayer. The third nitride semiconductor layer may be formed on the raisedportion. This structure mitigates reduction in the third nitridesemiconductor layer.

In the example nitride semiconductor device, the first ohmic electrodemay be a cathode electrode. The Schottky electrode may be an anodeelectrode.

In the example nitride semiconductor device, a bottom end of the anodeelectrode is lower than an interface between the first nitridesemiconductor layer and the second nitride semiconductor layer.

The example nitride semiconductor device may further include multipleones of the at least one first nitride semiconductor layer, and multipleones of the at least one second nitride semiconductor layer. The firstnitride semiconductor layers and the second nitride semiconductor layersmay be alternately formed. In this case, a bottom end of the anodeelectrode may be lower than an interface between a lowermost one of thefirst nitride semiconductor layers and a lowermost one of the secondnitride semiconductor layers.

In the example nitride semiconductor device, the anode electrode mayinclude a plurality of anode fingers connected in parallel to eachother. The cathode electrode includes a plurality of cathode fingersconnected in parallel to each other. The anode fingers and the cathodefingers may be alternately formed.

In this case, the third nitride semiconductor layer may surround each ofthe plurality of anode fingers. Also, the third nitride semiconductorlayer may discontinuously surround each of the plurality of anodefingers.

The example nitride semiconductor device may further include a secondohmic electrode located opposite to the first ohmic electrode relativeto the Schottky electrode. The first ohmic electrode may be a drainelectrode. The second ohmic electrode may be a source electrode. TheSchottky electrode may be a gate electrode.

In the example nitride semiconductor device, the second nitridesemiconductor layer may include a recess portion formed between thedrain electrode and the source electrode. The gate electrode may beformed to fill the recess portion.

In the example nitride semiconductor device, the second nitridesemiconductor layer may include a recess portion formed between thedrain electrode and the source electrode. The gate electrode and thethird nitride semiconductor layer may be formed in the recess portion.

In the example nitride semiconductor device, the third nitridesemiconductor layer may be formed to surround the gate electrode.

In the example nitride semiconductor device, the source electrode mayinclude a plurality of source electrode fingers connected in parallel toeach other. The drain electrode may include a plurality of drainelectrode fingers connected in parallel to each other. The gateelectrode may include a plurality of gate electrode fingers connected inparallel to each other. The source electrode fingers and the drainelectrode fingers are alternately formed. The gate electrode fingers areformed between the source electrode fingers and the drain electrodefingers.

In this case, the third nitride semiconductor layer may surround each ofthe plurality of gate electrode fingers. Also, the third nitridesemiconductor layer may discontinuously surround the gate electrodefingers.

According to the present disclosure, a nitride semiconductor device witha low leakage current can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a nitride semiconductordevice according to an embodiment.

FIG. 2 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 3 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 4 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 5 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 6 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIGS. 7A-7C illustrate a variation of the nitride semiconductor deviceaccording to the embodiment. FIG. 7A is a top view. FIG. 7B is anenlarged top view illustrating part of FIG. 7A. FIG. 7C is across-sectional view taken along the line VIIc-VIIc of FIG. 7B.

FIG. 8 is a top view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 9 is a cross-sectional view illustrating a variation of the nitridesemiconductor device according to the embodiment.

FIG. 10 is a cross-sectional view illustrating a variation of thenitride semiconductor device according to the embodiment.

FIG. 11 is a cross-sectional view illustrating a variation of thenitride semiconductor device according to the embodiment.

FIG. 12 is a cross-sectional view illustrating a variation of thenitride semiconductor device according to the embodiment.

FIGS. 13A-13C illustrate a variation of the nitride semiconductor deviceaccording to the embodiment. FIG. 13A is a top view. FIG. 13B is anenlarged top view illustrating part of FIG. 13A. FIG. 13C is across-sectional view taken along the line XIIIc-XIIIc of FIG. 13B.

DETAILED DESCRIPTION

In the present disclosure, AlGaN represents ternary compoundAl_(x)Ga_(1-x)N, where 0≦x≦1. Multi-element mixed crystal compounds areexpressed simply by the arrangement of chemical symbols such as AlInN,GaInN, etc. For example, nitride semiconductor Al_(x)Ga_(1-x-y)In_(y)N,where 0≦x ≦1, 0≦y≦1, and x+y≦1, is expressed simply by AlGaInN. Thewording “undoped” means that impurities are not intentionallyintroduced.

First Embodiment

FIG. 1 illustrates a cross-sectional structure of a nitridesemiconductor device according to an embodiment. As shown in FIG. 1, thenitride semiconductor device of this embodiment is a Schottky diode. Asemiconductor multilayer 102 made of nitride semiconductor is formed ona substrate 101. The semiconductor multilayer 102 includes a bufferlayer 121, a first nitride semiconductor layer 122 formed on the bufferlayer 121, a second nitride semiconductor layer 123 formed on the firstnitride semiconductor layer 122, and a third nitride semiconductor layer124 selectively formed on the second nitride semiconductor layer 123.The buffer layer 121 may be made of AN etc. with a thickness of about 2μm. The thickness and the material of the buffer layer 121 may bechanged as appropriate according to the type of the substrate 101.Alternatively, the buffer layer 121 may not be formed depending on thetype of the substrate. The first nitride semiconductor layer 122 may bemade of undoped GaN with a thickness of about 3 μm. The second nitridesemiconductor layer may be made of undoped AlGaN with a thickness ofabout 25 nm. The third nitride semiconductor layer may be selectivelyformed on the second nitride semiconductor layer 123 and made of p-typeAlGaN with a thickness of about 200 nm.

A cathode electrode 131 being an ohmic electrode and an anode electrode132 being a Schottky electrode are formed on the semiconductormultilayer 102. The cathode electrode 131 is formed of a multilayer filmof, for example, titanium and aluminum. The cathode electrode 131 isformed in a recess portion, which reaches a lower position than theinterface between the second nitride semiconductor layer 123 and thefirst nitride semiconductor layer 122 in direct contact with a 2DEGlayer. The cathode electrode 131 is spaced apart from the third nitridesemiconductor layer 124. If an ohmic contact can be formed with the 2DEGlayer, there is no need to form any recess portion. The anode electrode132 is formed on the second nitride semiconductor layer 123 and made ofSchottky metal. The anode electrode 132 is in contact with a sidesurface of the third nitride semiconductor layer 124 located opposite tothe cathode electrode 131. The Schottky metal may be, for example,nickel (Ni), palladium (Pd), or gold (Au). The Schottky metal may beformed of a multilayer of two or more of these materials.

The semiconductor multilayer 102 is covered by a passivation film 141made of silicon nitride (SiN) etc. In a conventional Schottky diode,when a passivation film is formed, a leakage current caused when areverse bias voltage has been applied increases by about 3 digits ascompared to the case where no passivation film is formed. This may bebecause a leakage path occurs at the interface between the passivationfilm and the semiconductor multilayer. However, in the semiconductordevice of this embodiment, the anode electrode 132 is located oppositeto the cathode electrode 131 relative to the p-type third nitridesemiconductor layer 124. The anode electrode 132 is in contact with thethird nitride semiconductor layer 124. Therefore, in the semiconductordevice of this embodiment, a leakage current at the time of applying areverse bias voltage can be maintained low, even when the passivationfilm 141 is formed.

Since the p-type third nitride semiconductor layer 124 servers as apotential barrier, the leakage path appearing at the interface betweenthe passivation film 141 and the second nitride semiconductor layer 123is blocked by the third nitride semiconductor layer 124. This largelyreduces leakage currents flowing through the interface between thepassivation film 141 and the second nitride semiconductor layer 123. Adepletion layer expands below the third nitride semiconductor layer 124.The 2DEG layer is narrowed and leakage currents at a Schottky junctioncan be reduced. As a result, in the nitride semiconductor device of thisembodiment, the leakage currents can be reduced by about 4 digits ascompared to a conventional nitride semiconductor device including apassivation film.

By forming the third nitride semiconductor layer 124, the breakdownvoltage rises as compared to a conventional nitride semiconductordevice. This may be because the third nitride semiconductor layer 124serves as a field plate to reduce electric field concentration at theSchottky junction.

On the other hand, when a forward bias voltage is applied to a Schottkydiode, the area of the depletion layer decreases, and the forwardcurrent flows to the Schottky junction. Thus, the forwardcharacteristics hardly changes as compared to the case where the thirdnitride semiconductor layer 124 is not formed.

In this embodiment, the anode electrode 132 is formed to cover part ofthe upper surface of the third nitride semiconductor layer 124. As such,if the anode electrode 132 is formed to overlap the third nitridesemiconductor layer 124, alignment is facilitated. However, the anodeelectrode 132 may have any structure as long as it is located oppositeto the cathode electrode 131 relative to the third nitride semiconductorlayer 124, and the third nitride semiconductor layer 124 has potentialequal to that of the anode electrode 132. However, it is not preferablethat the third nitride semiconductor layer 124 be in contact with thecathode electrode 131, since a current flows to the third nitridesemiconductor layer 124.

The anode electrode 132 is formed on the second nitride semiconductorlayer 123. However, as shown in FIG. 2, a recess portion may be formedto reach a lower position than the interface between the second nitridesemiconductor layer 123 and the first nitride semiconductor layer 122,and the anode electrode 132 may be formed in the recess portion. Thisstructure improves forward characteristics, since the Schottky metal isin direct contact with the 2DEG layer.

An example has been described where the second nitride semiconductorlayer 123 has a uniform thickness. As shown in FIG. 3, however, a raisedportion 123 a with a greater thickness than the other portions may beformed in the second nitride semiconductor layer 123, and the thirdnitride semiconductor layer 124 may be formed on the raised portion 123a. The carrier concentration of the 2DEG layer may decrease due toraising of the conduction band directly under the portion where thep-type third nitride semiconductor layer 124 is formed. By increasingthe thickness of the second nitride semiconductor layer 123 directlyunder the third nitride semiconductor layer 124, a decrease in thecarrier concentration of the 2DEG layer due to the third nitridesemiconductor layer 124 can be compensated and on-resistance can bereduced. When the second nitride semiconductor layer 123 made of AlGaNis formed, the thickness is generally about 25 nm. In this case, thethickness directly under the third nitride semiconductor layer 124 mayrange from about 30 nm to about 45 nm. This structure may be combinedwith the structure of the anode electrode 132 formed in the recessportion.

As shown in FIG. 4, the semiconductor multilayer 102 may include amulti-channel layer 126 formed by alternately stacking first nitridesemiconductor layers 122A made of undoped GaN and second nitridesemiconductor layers 123A made of undoped AlGaN.

When the multi-channel layer 126 is formed, the 2DEG layer serving as achannel is generated near the interface between the first nitridesemiconductor layer 122A and the second nitride semiconductor layer 123Ain each of the first nitride semiconductor layers 122A. Since aplurality of channels are provided, on-resistance can be largelyreduced. In this case as well, as shown in FIG. 5, the anode electrode132 may be formed in the recess portion and may be in direct contactwith the 2DEG layers. Also, as shown in FIG. 6, in the uppermost secondnitride semiconductor layer 123A, a raised portion 123 a with a greaterthickness than the other portions may be formed directly under theportion where the p-type third nitride semiconductor layer 124 isformed. While in FIGS. 4-6, two cycles of the first nitridesemiconductor layers 122A and the second nitride semiconductor layers123A are provided, three or more cycles may be stacked.

As shown in FIGS. 7A-7C, a multi-finger type diode may be formed, inwhich a plurality of cathode electrode fingers 131A and a plurality ofanode electrode fingers 132A are alternately arranged. FIG. 7Aillustrates the entire structure viewed from above. FIG. 7B is anenlarged top view of the structures of the cathode electrode fingers131A and the anode electrode fingers 132A. FIG. 7C illustrates across-sectional structure taken along the line VIIc-VIIc of FIG. 7B. InFIG. 7B, the passivation film 141, an interlayer insulating film 145,cathode interconnects 151, and anode interconnects 154 are not shown.

The cathode electrode fingers 131A are connected to a cathode pad 152 bythe cathode interconnects 151 formed on the interlayer insulating film145. The anode electrode fingers 132A are connected to an anode pad 155by the anode interconnects 154.

In FIGS. 7A-7C, the third nitride semiconductor layer 124 is formed tosurround each of the anode electrode fingers 132A. The depletion layerexpands not only directly under the third nitride semiconductor layer124, but obliquely downward. Thus, the third nitride semiconductor layer124 does not necessarily continuously surround each of the anodeelectrode fingers 132A, but may be discontinuously surround each of theanode electrode fingers 132A as shown in FIG. 8. This mitigates adecrease in the carrier concentration of the 2DEG layer due to the thirdnitride semiconductor layer 124. The intervals of the parts of thediscontinuous third nitride semiconductor layer 124 may be controlled sothat the depletion layers overlap each other. In FIGS. 7A-7C, an examplehas been described where the anode electrode fingers 132A are formed inthe recess portion which reaches a lower position than the interfacebetween the second nitride semiconductor layer 123 and the first nitridesemiconductor layer 122 similar to FIG. 2. However, the anode electrodefingers 132A may not necessarily include a recess structure. In FIGS.3-6, a multi-finger type diode may be formed.

As shown in FIG. 9, the present disclosure is not limited to a diode butmay be implemented as a transistor. In the case of a transistor, asource electrode 135 and a drain electrode 136 may be formed as ohmicelectrodes, and a gate electrode 137 may be formed as a Schottkyelectrode. The third nitride semiconductor layer 124 may be formed tosurround the gate electrode 137. This structure reduces leakage currentsbetween the source electrode 135 and the gate electrode 137, and leakagecurrents between the drain electrode 136 and the gate electrode 137.

In the case of a transistor, leakage currents between the drainelectrode 136 and the gate electrode 137 have been generallyproblematic. Thus, as shown in FIG. 10, the third nitride semiconductorlayer 124 may not be formed between the gate electrode 137 and thesource electrode 135. The second nitride semiconductor layer 123 mayhave an increased thickness directly under the third nitridesemiconductor layer 124.

As shown in FIG. 11, a recess portion may be formed in the secondnitride semiconductor layer 123 to be filled by the gate electrode 137.By forming this gate recess structure, current collapse can be reducedand a threshold voltage can be raised. Alternatively, as shown in FIG.12, a wide recess structure may be formed, in which the gate electrode137 and the third nitride semiconductor layer 124 are formed in a recessportion. In FIGS. 11 and 12 as well, the third nitride semiconductorlayer 124 may not be formed between the source electrode 135 and thegate electrode 137.

As shown in FIGS. 13A-13C, a multi-finger type transistor may be formed,in which a plurality of source electrode fingers 135A and a plurality ofdrain electrode fingers 136A are alternately arranged, and gateelectrode fingers 137A are formed between the source electrode fingers135A and the drain electrode fingers 136A. FIG. 13A illustrates theentire structure viewed from above. FIG. 13B is an enlarged top view ofthe structures of the source electrode fingers 135A, the drain electrodefingers 136A, and the gate electrode fingers 137A. FIG. 13C illustratesa cross-sectional structure taken along the line XIIIc-XIIIc of FIG.13B. In FIG. 13B, the passivation film 141, the interlayer insulatingfilm 145, source interconnects 161, drain interconnects 164, and gateinterconnects 167 are not shown.

The source electrode fingers 135A are connected to a source pad 162between the source interconnects 161 formed on the interlayer insulatingfilm 145. The drain electrode fingers 136A are connected to a drain pad165 by the drain interconnects 164. The gate electrode fingers 137A areconnected to a gate pad 168 by the gate interconnects 167.

The third nitride semiconductor layer 124 may not be formed by thesource electrode fingers 135A and the gate electrode fingers 137A. Thethird nitride semiconductor layer 124 may not be formed continuously,but may be divided into a plurality of parts which are arranged atpredetermined intervals.

The third nitride semiconductor layer may be made of GaN in place ofAlGaN as long as it is a p-type nitride semiconductor layer. However,the third nitride semiconductor layer preferably has a narrower bandgapthan the second nitride semiconductor layer. Thus, if the third nitridesemiconductor layer and the second nitride semiconductor layer are bothmade of AlGaN, the third nitride semiconductor layer preferably has alower Al composition ratio than the second nitride semiconductor layer.P-type impurities may be magnesium (Mg) etc. When the impurityconcentration is about 1×10¹⁷ cm⁻³ or more, the influence of a leakagepath appearing at the interface between the second nitride semiconductorlayer and the passivation film can be reduced. In order to effectivelyreduce the influence of the leakage path, it is preferably about 1×10¹⁸cm⁻³ or more. However, it may be about 1×10²⁰ cm⁻³ or less, since thecarrier concentration of the 2DEG layer decreases when the third nitridesemiconductor layer has a high impurity concentration. In order toreduce the influence on the 2DEG layer, it is preferably about 1×10¹⁹cm³ or less.

The width of the third nitride semiconductor layer may be determined asappropriate, as long as the influence of the leakage path appearing atthe interface between the second nitride semiconductor layer and thepassivation film can be reduced. Note that, if the width of the thirdnitride semiconductor layer is great, the area of the 2DEG layer, whichhas a reduced carrier concentration, becomes large. On the other hand,if the area is too small, the process margin is lost and the thirdnitride semiconductor layer is difficult to form. Therefore, the widthis preferably about 1 μm.

The first nitride semiconductor layer and the second nitridesemiconductor layer may be provided in any combination as long as a 2DEGlayer can be formed at the heterojunction interface.

While an example has been described where the passivation film is a SiNfilm, similar advantages can be provided by a passivation film of othermaterials. In place of the SiN film, the passivation film may be an AlNfilm, SiO₂ film, Al₂O₃, etc. The passivation film may be a multilayerfilm of an AlN film and a SiN film etc.

The substrate may be made of silicon (Si), silicon carbide (SiC),gallium arsenide (GaAs), gallium nitride (GaN), zinc oxide (ZnO),sapphire, etc.

According to the present disclosure, a nitride semiconductor device withreduced interface leakage currents is provided even when a passivationfilm is formed, and is thus particularly useful as a power device etc.in a power supply circuit.

1. A nitride semiconductor device comprising: a semiconductor multilayerincluding at least one first nitride semiconductor layer formed on asubstrate, at least one second nitride semiconductor layer formed on thefirst nitride semiconductor layer, and having a wider bandgap than thefirst nitride semiconductor layer, and a p-type third nitridesemiconductor layer selectively formed on the second nitridesemiconductor layer; a first ohmic electrode and a Schottky electrodespaced apart from each other on the semiconductor multilayer; and apassivation film covering a top of the semiconductor multilayer, whereinthe third nitride semiconductor layer is formed between the first ohmicelectrode and the Schottky electrode in contact with the second nitridesemiconductor layer and the Schottky electrode.
 2. The nitridesemiconductor device of claim 1, wherein the Schottky electrode is incontact with at least part of an upper surface of the third nitridesemiconductor layer.
 3. The nitride semiconductor device of claim 1,wherein the second nitride semiconductor layer includes a raised portionwith a greater thickness than the other portions of the second nitridesemiconductor layer, and the third nitride semiconductor layer is formedon the raised portion.
 4. The nitride semiconductor device of claim 1,wherein the first ohmic electrode is a cathode electrode, and theSchottky electrode is an anode electrode.
 5. The nitride semiconductordevice of claim 4, wherein a bottom end of the anode electrode is lowerthan an interface between the first nitride semiconductor layer and thesecond nitride semiconductor layer.
 6. The nitride semiconductor deviceof claim 4, further comprising: multiple ones of the at least one firstnitride semiconductor layer; and multiple ones of the at least onesecond nitride semiconductor layer, wherein the first nitridesemiconductor layers and the second nitride semiconductor layers arealternately formed.
 7. The nitride semiconductor device of claim 6,wherein a bottom end of the anode electrode is lower than an interfacebetween a lowermost one of the first nitride semiconductor layers and alowermost one of the second nitride semiconductor layers.
 8. The nitridesemiconductor device of claim 4, wherein the anode electrode includes aplurality of anode fingers connected in parallel to each other, thecathode electrode includes a plurality of cathode fingers connected inparallel to each other, and the anode fingers and the cathode fingersare alternately formed.
 9. The nitride semiconductor device of claim 8,wherein the third nitride semiconductor layer surrounds each of theplurality of anode fingers.
 10. The nitride semiconductor device ofclaim 9, wherein the third nitride semiconductor layer discontinuouslysurrounds each of the plurality of anode fingers.
 11. The nitridesemiconductor device of claim 1, further comprising a second ohmicelectrode located opposite to the first ohmic electrode relative to theSchottky electrode, wherein the first ohmic electrode is a drainelectrode, the second ohmic electrode is a source electrode, and theSchottky electrode is a gate electrode.
 12. The nitride semiconductordevice of claim 11, wherein the second nitride semiconductor layerincludes a recess portion formed between the drain electrode and thesource electrode, and the gate electrode is formed to fill the recessportion.
 13. The nitride semiconductor device of claim 11, wherein thesecond nitride semiconductor layer includes a recess portion formedbetween the drain electrode and the source electrode, and the gateelectrode and the third nitride semiconductor layer are formed in therecess portion.
 14. The nitride semiconductor device of claim 11,wherein the third nitride semiconductor layer is formed to surround thegate electrode.
 15. The nitride semiconductor device of claim 11,wherein the source electrode includes a plurality of source electrodefingers connected in parallel to each other, the drain electrodeincludes a plurality of drain electrode fingers connected in parallel toeach other, the gate electrode includes a plurality of gate electrodefingers connected in parallel to each other, the source electrodefingers and the drain electrode fingers are alternately formed, and thegate electrode fingers are formed between the source electrode fingersand the drain electrode fingers.
 16. The nitride semiconductor device ofclaim 15, wherein the third nitride semiconductor layer surrounds eachof the plurality of gate electrode fingers.
 17. The nitridesemiconductor device of claim 16, wherein the third nitridesemiconductor layer discontinuously surrounds the gate electrodefingers.